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Re: QT calculation - Converting frames to milliseconds, coding question

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Andreas KielRe: QT calculation - Converting frames to milliseconds, coding question
by on Mar 20, 2011 at 1:36:30 pm

This is an interesting thread. Her my 2cents.

I think the first question from Ariane was a bit misleading as she mixed up 24 NTSC and 24.
Otherwise it's very simple.
For NTSC time the calulation has to be ((20*24+5)/24)*(1001/1000) -> 20.22854166666. So the media info from the server is correct.
If there will be true 24, the calculation must be (20*24+5)/24) -> 20.20833333333

I'm not sure what Ariane meaned by "When I open it in QT...". With QT Player Pro I get displayed 20.22 - which is rounded down, but more or less correct. But also 20.27 is correct it just depends on way how you look at it. This time takes the time of the frame after the last frame, so it includes one frame more to calculate with results in 20.27.

If you do it with QT code it will give you the same results.
To address a frame's time for example the last (485 * 1000)/23976 -> 20.2285618952 this includes some rounding error as 23976 is rounded.

In this case 1000 is the internal frame duration of this special example QT movie. The values used apply to any QT file created with FCP -- except the frame number.
For 29.97 the frame duration will be 100 and timebase is 2997, for PAL and other non NTSC movies frame duration is always 100 and timebase is 100*FPS - with FCP QT movies.

Other apps might use a different timebase. An old fashioned one is 600 as it can be divided by 12 and 15 for stop motion, by 24 for film and 30, 60 for NTSC non drop.
So to calculate time from an unknown movie you have to retrieve and verify timebase and frame duration.

Andreas

Spherico
http://www.spherico.com/filmtools


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